In the prior art, so-called Ziegler-Natta catalyst consisting of a titanium or vanadium compound as a primary catalyst component and an alkylaluminum compound as cocatalyst component have been generally used for preparing ethylene homopolymers or copolymers of ethylene and α-olefin. Although a Ziegler-Natta catalytic system exhibits high activity on ethylene polymerization, the catalytic system has disadvantages in that molecular weight distribution of the produced polymer is broad due to non-uniform catalyst activation point, and especially, composition distribution thereof is not uniform in the copolymers of ethylene and α-olefin.
Recently, so-called metallocene catalytic systems consisting of a metallocene compound of Group 4 transition metal in the Periodic Table of Elements, such as titanium, zirconium and hafnium, and methylaluminoxane as a cocatalyst have been developed. The metallocene catalytic system is a homogeneous catalyst having a mono-modal catalyst activation point, and thus, can provide prepare polyethylene having narrower molecular weight distribution and more homogenous composition distribution as compared with the existing Ziegler-Natta catalyst system. For example, European Patent Laid-Open Publication Nos. 320,762 and 372,632; Japanese Patent Laid-Open Publication Nos. Sho 63-092621, Hei 02-84405 and Hei 03-2347 reported that ethylene may be polymerized with high activity by activating metallocene compounds such as Cp2TiCl2, Cp2ZrCl2, Cp2ZrMeCl, Cp2ZrMe2, ethylene(IndH4)2ZrCl2 by using methylaluminoxane as a cocatalyst, to prepare polyethylene having a molecular weight distribution (Mw/Mn) in the range from 1.5 to 2.0. However, it is difficult to obtain high-molecular weight polymers by using the above catalytic system, and further, when solution polymerization executed at a high temperature of 100° C. or higher is employed, polymerizing activity abruptly decreases and β-dehydrogenation is predominant. Therefore, the system has been known to be not suitable for preparing high-molecular weight polymers having a weight average molecular weight (Mw) of 100,000 or more.
Meanwhile, there was reported so-called geo-restrictive non-metallocene based catalysts (also referred to as single activation point catalysts) where the transition metals are linked in a ring type, as catalysts for preparing high-molecular weight polymers with high catalytic activity in ethylene homopolymerization or copolymerization of ethylene and α-olefin in the solution polymerization conditions. European Patent Nos. 0416815 and 0420436 suggest an example where amide group is linked to one cyclopentadiene ligand in a ring type, and European Patent No. 0842939 shows an example of the catalyst where phenol-based ligand as an electron donor compound is linked to cyclopentadiene ligand in a ring type. This geo-restrictive catalyst may remarkably improve reactivity with higher α-olefins due to lowered sterical hindrance effect of the catalyst itself, but has many difficulties in the commercial use thereof. Therefore, it has been important to secure more competitive catalytic systems in requiring commercialized catalysts based on economical feasibility, that is, excellent high-temperature activity, excellent reacitivity with higher α-olefins, and capability to prepare high-molecular weight polymers.